Trump's deadline to rule out a third-party bid: Sept. 30

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Buisnessman Donald Trump speaks during the Freedom Summit on May 9, 2015 in Greenville, South Carolina. Trump joined potential presidential candidates in addressing the event hosted by conservative group Citizens United.

Washington (CNN)Donald Trump must rule out a third-party bid before October if he wants to compete in South Carolina's Republican primary, a crucial test in the nominating contest.

Trump has repeatedly refused to rule out a third-party candidacy, noting that he could use the threat of an independent bid as leverage, but he cannot appear on the South Carolina primary ballot unless he pledges to support the GOP nominee in the general election.

Trump said Tuesday when asked about the rule by reporters in Iowa that his campaign is "looking into it."

"We certainly have plenty of time," he said. "We're leading every poll, we're leading every state, from Iowa to New Hampshire to South Carolina, polls have come in from virtually every place ... so my whole desire is just fairness, and I want to run as the Republican nominee, I want to win, I think we will win."

Candidates must file their candidacy with the South Carolina GOP by Sept. 30, submitting a form that includes a signed pledge stating to "hereby affirm that I generally believe in and intend to support the nominees and platform of the Republican Party in the November 8, 2016 general election."

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South Carolina GOP chairman Matt Moore told CNN Tuesday that the party would consider legal action against any candidate who signs and later violates the pledge, which Moore said the state party has required for "decades."

"It has not been an issue with national candidates in the past," Moore said. "We're hoping it's not this time around."

Trump is the only Republican presidential candidate to refuse to pledge not to launch an independent campaign if he cannot secure the GOP's presidential nomination -- a fact that was put into relief during the first GOP primary debate earlier this month when Trump was the only candidate to raise his hand when the moderators raised the question at the top of the debate.